Water craft of various types have been used for centuries. For some types of watercraft, such as inflatable boats and rafts, portability and ease of storage are primary factors to be considered when designing the craft. One way of improving portability and ease of storage is to reduce the overall size and weight of the craft. Reduction of the overall size and weight can generally be accomplished by reduction in the size and weight of the various components or members of the craft.
One member which is common to many boats is a keel. For many water craft the purpose of the keel is to act as a primary structural support. Another purpose, in many craft, is to provide stability by making it more difficult for the craft to be pushed "sideways" to the line of travel, thus making it easier to propel the craft in a straight line, and to provide a countering force. Additionally, the keel in many craft is used to shift the center of gravity downward to increase the probability that the craft will remain upright. Thus, keels are frequently designed to be rigid, heavy, or both rigid and heavy. Providing a craft with a keel which is rigid or heavy tends to make it less portable and more difficult to store.
When designing water craft, it is generally desirable to reduce drag so that less energy is required to accelerate the craft to some velocity, and to maintain that velocity once it has been reached. Inflatable rafts frequently have flat bottoms to reduce drag at lower speeds and to permit hydroplaning at higher speeds. If the raft is self-propelled, hydroplaning at higher speeds is generally desirable. However, if the raft is being towed, the tendency to hydroplane is less desirable as it frequently results in instability with the raft bouncing about and wandering back and forth in the wake of the craft which is towing the raft.
Inflatable craft are frequently designed to be both portable and easily storable. As such, it is generally undesirable to provide such a craft with a rigid or heavy keel. As a result, various alternatives have been tried.
One such alternative is the use of fins, center boards, dagger board, and outriggers (see U.S. Pat. Nos. 3,577,576 to Lobb (May 4, 1971), 4,249,276 to Snyderman (Feb. 10, 1981), and 4,735,163 to Filshie (Apr. 5, 1988)). However, such devices are not completely satisfactory as their use requires providing a means for mounting them to the craft, thus increasing the cost of the craft, and increasing the time required for setting up the craft. Additionally, being rigid and having more weight than the inflatable portions of the craft, they tend to decrease portability and ease of storage. Portability and ease of storage is also decreased because of the increased number of parts.
Another alternative to providing a rigid and heavy keel is to modify the bottom or hull of the craft into a `V` shape. This alternative is generally used with craft having substantially rigid bottoms. As an example, U.S. Pat. No. 3,694,836 discusses a collapsible boat having a substantially rigid buoyant bottom. However, because of the substantially rigid bottom, the craft, even when collapsed, occupies at least as much space as the bottom. Another example having a similar problem is U.S. Pat. No. 4,858,550 which discusses the use of a hard shell shaped to cover the bottom of an inflatable craft and made from various contoured staves or segments joined together. Although there is less of a problem in regard to size as the segments can be separated, the individual segments are still of a size and weight to limit portability and ease of storage. Also, as with the use of devices such as dagger boards, there are more items to maintain, increased cost, and increased setup time.
For craft having a bottom comprising a hard, flat, and rigid surface covered by a more watertight, flexible material, hull shape modification can be accomplished using a cylindrical, inflatable member between the rigid bottom and the flexible sheet. U.S. Pat. Nos. 4,640,217 to Ferronmiere (Feb. 3, 1987) and 4,603,651 to Harding (Aug. 5, 1986) discuss craft having such members. Two of the primary problems with these designs are that (1) the `V` shape of the hull tends to be relatively flat and to have a rounded bottom, thus decreasing the ability to cut into the surface of the water and prevent hydroplaning, and (2) a hard, flat, and rigid bottom is required to provide the support required for the hull to maintain its shape. The shaped hulls of Ferronmiere '217 and Harding '651 don't comprise an "inflatable keel" as the term is used herein in that the hulls themselves are not inflatable, the inflatable members used to modify the shape of the hulls don't contact the water, and a rigid bottom is required in conjunction with the inflatable member to properly shape the hull.
Thus there is a continuing need for improvements in the design of water craft.